Redirect to external issue tracker from `/issues`
Prior, in order to display the correct link to "Issues" in the project
navigation, we were performing a check against the project to see if it
used an external issue tracker, and if so, we used that URL. This was
inefficient.
Now, we simply _always_ link to `namespace_project_issues_path`, and
then in the controller we redirect to the external tracker if it's
present.
This also removes the need for the `url_for_issue` helper. Bonus! 🎉
See merge request !5608
b69cc5239d